A _______________ permanently joins two lengths of pipe together.

The correct choice, coupling, refers to a fitting that connects two lengths of pipe together in a permanent manner. These fittings enable a secure and leak-proof connection, ensuring that the flow of liquid or gas through the pipes is uninterrupted and that the integrity of the piping system is maintained. Couplings are essential in various plumbing and piping applications where a seamless connection is needed, and they are designed to withstand pressure and environmental conditions.

In contrast, a clamp typically provides a temporary or adjustable hold rather than a permanent connection. A joint usually refers to the point where two pieces meet but does not specifically describe a component that permanently connects them. A sleeve can be used to cover or protect a portion of the pipe but may not ensure a permanent bond between two pipe lengths. Thus, coupling is the most accurate term for the context provided.